Although Pardridge et al. demonstrated that BDNF in blood was rapidly degraded and no mBDNF transcytosis through the blood–brain barrier (BBB) in rat was observed [139], later studies showed that blood-borne mBDNF can enter the CNS by a rapid, saturable transport system of the BBB [140,141], mBDNF concentration in blood is reduced in psychiatric and neurological disorders [142], and physical exercise is an effective means to alleviate the mental dysfunction in a lot of psychiatric diseases [143]. This evidence concerns the gene BDNF and psychiatric disorder.